The present application relates to an improved ballast control circuit. In particular, the present application relates to an improved ballast control circuit for use in controlling power supplied to one or more CCFL or EEFL lamps.
Most liquid crystal display (LCD) screens and monitors use Cold Cathode Fluorescent Lamp (CCFL) backlighting. Typically, a number of CCFL lamps, which are typically long and thin in shape, are arranged in the LCD in a row to provide backlighting for the screen or monitor. It is important that this backlight have an even intensity in order to ensure that the image on the LCD screen or monitor is properly displayed.
High frequency electronic ballasts are typically used to provide the voltage and power necessary to correctly ignite and supply the lamps. A single ballast is preferably able to power all of the lamps. The ballast should meet certain other criteria as well. The ballast should have a fixed frequency of operation in order to prevent the occurrence of interference patterns on the screen, which may result from interaction between the image scanning frequency and the ballast frequency to produce beat frequencies. Further, the brightness of the lamps should also be controllable, and thus, the ballast must allow for dimming. The dimming method employed for CCFL lamps is preferably PWM burst mode dimming in which the high frequency ballast current driving the lamp is adjusted to control the length of the burst of high frequency current applied to the lamps, and thus, to control the brightness as a function of the RMS current. The frequency of the PWM control signal should be orders of magnitude lower than the ballast frequency, but high enough to prevent any noticeable flicker of the lamps. In addition, the ballast should include fault detection and shutdown features and a designated start up procedure to provide proper control when power is initially applied to the lamps.
The preferred ballast topology for use in CCFL backlighting applications is a half bridge, as is commonly used in general purpose fluorescent ballasts. Such ballast circuits are commonly controlled utilizing a single ballast control circuit which is commonly implemented as an integrated circuit (IC).
Accordingly is desirable to provide a ballast control circuit that meets the requirements set forth above.